When exposed to radiation such as X-rays, an energy-storing phosphor (e.g., stimulable phosphor, which gives off stimulated emission) absorbs and stores a portion of the radiation energy. The phosphor then emits stimulated emission according to the level of the stored energy when exposed to electromagnetic wave such as visible or infrared light (i.e., stimulating light). A radiation image recording and reproducing method utilizing the energy-storing phosphor has been widely employed in practice. In that method, a radiation image storage panel, which is a sheet comprising the energy-storing phosphor, is used. The method comprises the steps of: exposing the storage panel to radiation having passed through an object or having radiated from an object, so that radiation image information of the object is temporarily recorded in the panel; sequentially scanning the panel with stimulating light such as a laser beam to emit stimulated light; and photoelectrically detecting the emitted light to obtain electric image signals. The storage panel thus treated is subjected to a step for erasing radiation energy remaining therein, and then stored for the use in the next recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly used.
The radiation image storage panel (often referred to as energy-storing phosphor sheet) has a basic structure comprising a support and a phosphor layer provided thereon. However, if the phosphor layer is self-supporting, the support may be omitted. Further, a protective layer is normally provided on the free surface (surface not facing the support) of the phosphor layer so as to keep the phosphor layer from chemical deterioration or physical damage.
Various kinds of energy-storing phosphor layers are known. For example, the phosphor layer can comprise a binder and energy-storing phosphor particles dispersed therein, or otherwise can comprise agglomerate of an energy-storing phosphor without binder. The latter layer can be formed, for example, by a gas phase-accumulation method, in which a phosphor or material thereof is vaporized or sputtered so that the phosphor may be deposited and accumulated on a substrate to form a layer of the phosphor in the form of columnar crystals. The phosphor layer thus formed by a gas phase-accumulation method contains no binder and consists of only the phosphor, and there are gaps among the columnar crystals of the phosphor. Because of the gaps, the stimulating light can stimulate the phosphor efficiently and the emitted light can be collected efficiently, too. Accordingly, a storage panel having that accumulated phosphor layer has high sensitivity. At the same time, since the gaps prevent the stimulating light from diffusing parallel to the layer, the storage panel can give a reproduced image of high sharpness.
The radiation image recording and reproducing method (or radiation image forming method) has various advantages as described above. However, it is still desired that the radiation image storage panel used in the method have as high sensitivity as possible and, at the same time, give a reproduced radiation image of as high quality (in regard to sharpness and graininess) as possible.
JP-A-2003-050298 discloses a process for preparation of a radiation image storage panel. In the disclosed process, first a subbing layer made of a phosphor matrix compound in the form of columnar crystals is formed on a support by a gas phase-accumulation method, and then a phosphor layer composed of the matrix compound and an activator in the form of columnar crystals is formed on the subbing layer (so that each columnar crystal of the phosphor can grow on each columnar crystal of the phosphor matrix compound), to prepare a phosphor layer excellent in columnar crystallinity. The columnar phosphor crystals in the formed phosphor layer are partly fused and combined with the matrix columnar crystals in the subbing layer.
It has been found that, if the phosphor comprises an additive (such as activator) and a matrix compound in a particular combination (particularly, alkali metal halide stimulable phosphors such as CsBr:Eu), the phosphor layer directly formed on a support (or substrate) by a gas phase-accumulation method is often poor in adhesion to the support and hence is liable to separate from the substrate. Particularly, if the phosphor is vaporized, deposited and accumulated under a medium vacuum (0.1 to 10 Pa) by means of a resistance heater, the formed phosphor layer is poor in adhesion to the support.